This invention generally relates to the use of microorganisms to enhance oil recovery from petroleum reservoirs. Here, a specific strain of bacteria is used to plug high permeability zones and increase waterflood efficiency by redirecting the flood to lower permeabiltiy, oil-bearing zones.
Petroleum that is in underground reservoirs is brought to the surface in a variety of ways. One of the more notable publicly held ideas of oil recovery is the "gusher," however, due to the changing nature of oil reserves, and economic and environmental policies, the gusher is the thing of the past. Surface pumps, which are a common highway sight, oftentimes provide the lift force necessary to bring oil to the surface in those reservoirs where the reservoir pressure is insufficient. Additionally, subsurface pumps can be coupled with the surface pumps to assist in the lifting duty. However, there comes a time in the life of many reservoir formations in which the reservoir pressure and the pumping devices are not enough to overcome the oil viscosity and the capillary forces of the formation. At this point, enhanced oil recovery (EOR) techniques are useful to recover additional oil that refuses to come to the surface by the means described above.
The term "EOR" spans a panoply of techniques and devices that are used to recover the last bit of oil reserves. There are devices and methods for: steam injection, water injection, gas drive, emulsification, injecting plugging agents, etc. One "device" that may perform many of these feats is a microorganism, most notably, a bacteria.
The idea of using bacteria to increase or enhance oil recovery is not new. Many laboratory investigations and a number of field tests have been performed both in the U.S. and elsewhere [see generally, J. Davis, Petroleum Microbiology (1967) and works collected in J. E. Zajic et al., Microbes and Oil Recovery, Bioresource Publications, El Paso (1985)]. Several technical meetings devoted exclusively to microbial enhanced oil recovery (MEOR) have been held. Some of the previous literature consists of anecdotal accounts or inadequately controlled studies, resulting in a skeptical appraisal of the technology. [See also D. Hitzman, Petroleum Microbiology and the History of its Role in Enhanced Oil Recovery, Proc. Int'l. Conf. on Micro. Enh. Oil Rec., p 162, (May 16-21, 1982), and E. Donaldson et al., There are Bugs in My Oil Well, Chemtech, p 602 (October 1985).]
The principle behind MEOR is based on the fact that microbes can produce most of the agents now employed in chemical EOR; i.e., water-soluble polymers, surfactants, co-surfactants and solvents such as ethanol and acetone, and acids. (See M. Singer, Microbial Biosurfactants in Zajic, Microbes and Oil Recover, U.S. Pat. No. 4,522,261 to McInerney et al., U.S. Pat. No. 2,807,570 to Updegraff and U.S. Pat. No. 2,660,550 to Updegraff et al.) Some microbial-produced products, e.g., xanthan biopolymer, are now commercially used for EOR. Such use is dependent on the cost-effectiveness of the microbial product compared to competing non-microbial products, e.g., xanthan as compared to polyacrylamide. In this application, the definition of MEOR applies to processes involving the in-situ application of microbial processes and usually excludes EOR processes which merely involve the use of chemical products which are produced in a fermentation plant.
The specific application of microorganisms for EOR in this invention is their use for the selective plugging of zones of high permeability (i.e., thief zones) in petroleum reservoirs. To back up a bit, when water injection is used to recover oil, it is injected downhole in an injection well to move any oil out of the formation to be recovered at a producing well. The water pushes the oil out of the small interstices and pores of the rocks, but it pushes the oil out of the wider spaces and larger pores (i.e., zones of higher permeability) first, leaving the smaller areas still filled with oil. Since petroleum is formed in stratified sedimentary deposits, several distinct layers of oil-bearing sands are usually present over the vertical profile of an oil well. Different layers can vary widely in permeability and porosity, as well as other propeties. Since a waterflood will naturally seek the zone of least resistance (or highest permeability) low permeability zones may be bypassed. After a time, recoverable oil is "watered out" of the high permeability zones, but the low permeability streaks still contain considerable recoverable oil. "Profile modification" provides a way that the residual oil may be recovered from these lower permeability zones. Current technology involves the injection of water-soluble polymers, which selectively enter the high permeability zones. Cationic cross-linking agents, i.e., Cr.sup.+3, Ti.sup.+4, or Al.sup.+3, held in solution by a complexing agent (i.e., citrate) or by oxidation state, are co-injected with the polymer or are swept after the polymer. (See U.S. Pat. No. 4,552,217 to Wu et al.) As the polymer gradually cross-links and gels into a water-insoluble 3-D matrix in the high permeability zones, the waterflood is channeled into zones of lower permeability, thus increasing oil production. There are problems associated with the techniques of profile modification with cross-linking polymers. Such polymers are relatively expensive, they may shear-degrade upon injection at the wellhead and may not penetrate sufficiently before gelling. For this reason, the use of microorganisms may prove promising in profile modification because it may eliminate some of these problems.
As with other techniques, using microbes to plug high permeability zones is not exactly new. Some early researchers are listed in J. Davis, "Petroleum Microbiology" (1967) and more recently there is U.S. Pat. No. 4,558,739 to McInerney et al.; and D. Revus, A Study of Reservoir Selective Plugging Utilizing In Situ Growth of Bacteria to Improve Volumetric Sweep Efficiency, Masters Thesis, Univ. of Oklahoma (1982), P. Kalish et al., The Effect of Bacteria on Sandstone Permeability, 16 Jour. Pet. Tech. 805 (July 1964). C. Brierly et al., Investigation of Microbially Induced Permeability Loss During In-Situ Leaching, Bureau of Mines (NTIS Publication) (April 1982). They use microbes in a variety of ways to enhance oil recovery. Some researchers have used the bacteria that naturally exists in the formation and have simply injected nutrients downhole to stimulate their growth and plug the formation [see U.S. Pat. No. 4,475,590 to Brown and L. Allison, Effect of Microorganisms on Permeability of Soil Under Prolonged Submergence, 63 Soil Science 439 (1947)]. Others have injected bacteria downhole followed by a nutrient solution. Some researchers depend on the biomass of the bacteria for plugging purposes, while others show that exopolymers produced by the bacteria are effective in closing off areas of high permeability.
Another factor in this plugging technique is the size of the organism being injected. Smaller bacteria may penetrate the formation a bit easier than larger bacteria. To that end, the spores of different bacteria may be used for injection to penetrate even deeper. Spores penetrate a reservoir formation easier and become lodged in these permeable zones, so that when they are stimulated to grow by a nutrient solution, they will plug more pores more effectively.
Other problems exist with the nutrient solutions used in the prior art. For example, downhole in a petroleum reservoir, there are conditions that put further constraints on microorganisms. More specifically, connate water, in many formations, has both high concentrations of salt (NaCl), alkaline earth ions (Ca.sup.+2, Mg.sup.+2, Ba.sup.+2), and heavy metal ions. Such ions can form insoluble precipitates with many of the standard components of bacterial nutrient media. These precipitates can plug the wellbore and prevent injection of cells or nutrients. Furthermore, some of these ions are inhibitory or toxic or microbial cells and some (e.g., Ca.sup.+2) are inhibitory to production of microbial biopolymer. Bacteria and the nutrient source that are injected downhole must be tolerant to these if they are to survive.
The downhole environment is usually anoxic, unlike the highly oxygenated condition above. To be able to survive and live in both environments, a bacteria must either be shielded from oxygen (which may be difficult and expensive) or must be tolerant to it (e.g., a facultative anaerobe). [Bacteria can be broadly divided into 3 categories based on their ability to utilize and tolerate oxygen: (1) obligately aerobic bacteria, which require molecular oxygen for growth; (2) obligately anearobic bacteria, to which molecular oxygen is toxic; and (3) facultative anaerobes, which can grow either in the presence or absence of atmospheric oxygen. Of the three, facultative anaerobes appear to be the most suitable MEOR candidates, since they can survive exposure of air during storage and injection while retaining the ability to grow well anaerobically.]
The most important ingredient, i.e., the bacteria, sometimes must be selected for these exact conditions that exist in a reservoir. Also, the nutrient's solution has to be tailored to both the bacteria and the reservoir in which it will be injected. All these considerations must be merged together to provide the desired result in plugging the formation thief zones.